1. Field of the Invention
This invention relates to liquid crystal display devices and more particularly, to an improvement in alignment layers for liquid crystal used in such devices.
2. Description of the Prior Art
For obtaining good display characteristics in liquid crystal displays, it is necessary that the liquid crystal be aligned in a given direction and mono-domainized. For this purpose, the molecules of liquid crystal have to be aligned in one direction by the interfacial effect. A number of studies have been heretofore made on the alignment of liquid crystal. Typical liquid crystal alignment methods include an oblique deposition method and a rubbing method.
In the oblique deposition method, an oxide such as SiO is vacuum deposited obliquely with respect to the surface of substrate in a thickness of several hundred to several thousand angstroms to form an alignment layer for liquid crystal. If the angle of the deposition relative to the substrate plane is small, the molecules of liquid crystal are aligned obliquely. At this time, the long axis of the respective molecules is arranged along the direction of the deposition beam. On the other hand, when the deposition angle is sharp, the angle of inclination or pretilt angle of the liquid crystal molecules becomes substantially zero and the molecules are aligned homogeneously. The alignment of the molecules is such that the direction of the long axis of the molecules makes a right angle with the direction of the deposition beam. The specific alignment of the molecules realized by the oblique deposition technique is considered to result from the interaction of the ripple pattern of the layer formed on the substrate by the vacuum deposition and the liquid crystal molecules. The minute difference in the ripple pattern will produce the difference between the parallel alignment and the oblique alignment.
On the other hand, the rubbing method is an alignment treatment where an organic polymer layer or film formed on substrate is rubbed in a given direction. The reason why the molecules of liquid crystal are aligned by the influence of the rubbed layer are not fully clarified. Presumably, the alignment is considered to occur in the following manner: when the alignment film is rubbed so that the shear stress is applied onto the surface of the film, the polymer chains in the vicinity of the surface are arranged along the direction of the rubbing; and the liquid crystal is aligned along the arrangement of the polymer chains. The polymers ordinarily used as the alignment film are linear polymers such as polyimides, polyvinyl alcohol and the like. The pretilt angle of liquid crystals is, at most, from 0 to several degrees.
However, these oblique vacuum deposition and rubbing techniques have the following drawbacks.
With the oblique deposition technique, it is difficult to obtain a uniform alignment layer with a large area. Where a substrate to be deposited has a large deposition area, the angle of the deposition becomes different at edge and central portions of the substrate. This entails the difference in pretilt angle of liquid crystal. In addition, the deposition apparatus is necessary with high product costs.
According to the rubbing technique, a display element can be fabricated more easily than that obtained by the oblique deposition technique. The rubbing technique has been widely used for alignment of conventional display elements using nematic liquid crystals or so-called twisted nematic systems. However, with super twisted nematic (STN) liquid crystal displays which aim at the large area and high contrast of the display, the alignment with rubbing is not satisfactory in that the pretilt angle of liquid crystal is small, so that a hysteresis phenomenon is involved in voltage-transmission characteristic. Thus, an adequate threshold value cannot be obtained. Especially, with display elements using ferrodielectric liquid crystal, the pretilt angle is from zero to several degrees which is smaller than those attained by the oblique deposition technique. In addition, a disinclination position called zigzag defect is produced, making it difficult to obtain a high contrast.
Although the oblique deposition and rubbing techniques have, respectively, the merits, the problems as set out above are involved, making it very difficult to inexpensively obtain liquid crystal display devices having good characteristics.